How to make custom bump maps
I'm going to assume here that you have a high resolution image of the wall you want to replicate. I'm going to be using this texture from cgtextures.com.

The best way that I know of for this, is to import it into gimp, crop it to a 1:1 aspect ratio:



You then use the make seamless filter to get this result:



Then import it into Blender and use the bump node to generate a normal map.



Please note that this is how I like to setup a material, but the only important parts are the image texture and the bump nodes.